This invention relates to a structure realizing a high-efficiency light emission, in a thin-film light emitting display panel. This invention is utilized in light sources, information display panels, etc.
“Interference” (resonance) is a phenomenon of vibration obtainable as a result of superposition of a plurality of vibrations having an interfering character; and the term “vibrator” means an apparatus or a mechanism creating an interference (vibration).
By providing a semi-transparent reflecting mirror in the front of the light emitting plane of an organic light emitting element and making a vibrator (micro-vibrator) in which the optical length (the sum of forward and backward optical lengths) is equal to an integral times as long as the desired light-emitting wavelength, it is possible to make the light-emitting spectrum monochromatic and, at the same time, enhance the light emission peak strength (cf. “Organic Electric Field Light Emitting Element and Substrate Thereof” mentioned in JP-A-H08-213174).
FIG. 6A illustrates one example of the element structure thereof; wherein 101 is a semi-transparent reflecting film, 102 is a transparent electrically conductive film, 103 is a hole transporting layer, 104 is a light emitting layer, 105 is an electron transporting layer, 106 is an electron injecting layer such as an alkali metal compound or the like, and 107 is an anode made of aluminum or the like. The physical properties relating to the structure of vibrator are mentioned in detail in a document, T. Nakayama: “Organic luminescent devices with a microcavity structure”, included in “Organic electroluminescent materials and devices”, edited by S. Miyata, published by Gorden & Breach Science Publisher (1997).
For the purpose of realizing a “transparent light emitting panel”, a transparent element structure using a transparent electrode in place of an opaque metallic electrode has been proposed in JP-A-2002-231054, etc. FIG. 6B illustrates one example of the element structure, wherein 102A and 102B are transparent electrically conductive films, 103 is a hole transporting film, 104 is a light emitting layer, 105 is an electron transporting layer and 106 is an electron injecting layer such as an alkali metal compound or the like.
In a vibrator structure element for use in high-luminescence light emission, it is important to design optimum electric and magnetic field distributions in the direction of film thickness in the element, in order to realize a high luminance and a high efficiency (cf. Spring Meeting of the Japanese Society of Applied Physics, a-PB-11; etc.). However, since prior vibrator structure elements have hardly had a degree of freedom with regard to adjustment of film thickness of electron transporting layer because of the charge balance control of element, it is hardly possible to control the phase of electromagnetic wave emitted from the light emitting layer, reflected on the metallic electrode and again returned to the light emitting layer. As for the transparent element structure, optimum design of electromagnetic field distribution has not been performed, because a transparent element originally has no structure of returning the emitted light to inside of element and causing vibration.
In the electric field light emission utilizing the π electron light emission, the molecules used for light emission can be classified into two groups according to the excited state which the molecules use mainly for light emission. The first group consists of molecules utilizing the singlet excited state which are characterized in that (1) the internal quantum yield does not exceed 25% and (2) relaxation time of excited state (the characteristic time required for lowering the light emission strength by 1/e) is short (not longer than 100 ns).
The second group consists of molecules utilizing the triplet excited state, too, for the light emission. This group is characterized in that (1) the internal quantum yield exceeds 25%, (2) relaxation time of excited state is long (1 μs or more), and (3) the molecules are combined with (coordinated to) a heavy metal causing an orbital-spin interchanging interaction such as iridium, platinum and the like.
Some materials of the second group of which lifetime of relaxation of the excited state is so long as at least several μ seconds are transferred and diffused from the light emitting layer until the excited state is relaxed. Accordingly, when such a light emitting material is used, there arises a problem that, so far as an element of prior vibrator structure is used, the excited state having reached the metallic electrode is inactivated without light emission, so that high luminance and high efficiency cannot be realized.
The principle of light emission from an organic electric field light emitting element is as follows. Thus, an electric field is applied between one pair of electrode films to inject electrons and positive holes into the light emitting layers, recombining the electrons and the holes in the light emitting layer to form excitons, and light is emitted from the light-emitting molecules in the light emitting layer by the use of the excitons. As has been mentioned above, the light emitting layer used in the organic electric field light emitting element is formed from a plurality of organic thin films and the film thickness thereof is at most about several tens nm. Thus, herein is a problem that when lifetime of the exciton formed is long and the exciton is transferred over a long distance until it is diminished and it has reached the metallic electrode film, the exciton is diminished without taking part in the light emission and thereby reduces light emission efficiency of the element. In order to solve this problem, an electrode material not causing the disappearance of exciton before participation in light emission should be used, or the metallic electrode film should be sufficiently kept away from the region in which the exciton moves around.